Organometallics 1993,12, 1386-1393
1386
Insertion of Alkynes into the Pd-C Bond of Palladacycles. Mechanistic Information from High-pressure Kinetic and X-ray Structural Data Alexander D. Ryabov’J
Division of Chemistry, G. V.Plekhanov Russian Economic Academy, Stremyanny per. 28, 113054 Moscow, Russia
Rudi van Eldik’ Institut ffir Anorganische Chemie, Universitlit WittenlHerdecke, Stockumer, Strasse 10, 5810 Witten, Germany Guy Le Borgne
Laboratoire de Cristallochimie, Universitb de Rennes, URA 254 du CNRS, 35042 Rennes Cbdex, France
Michel Pfeffer’ Laboratoire de Synthbes Mbtallo-induites, Universitb Louis Pasteur, URA 416 du CNRS, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France Received October 23, 1992 The reaction between halo-bridged ring-substituted orthopalladated N,N-dimethylbenzylamines, [Pd(CsH&CHzNMez)Xl2 1(R = 4-Me0,5-Me, H, 5-F,X = C1, I) and alkynes R ’ C d R ” (R’IR’’ = Ph/Ph, Ph/CsH&F3-3, Ph/CsH4N02-4, and Et/Et) to afford double insertion products 3 is first-order in both the reagent concentrations in chloroform at 25.e44.1 OC. The insertion of the first alkyne molecule is the rate-determining step. Electron-donating groups R on the phenyl ring of 1 increase the rate constants and the slope of the Hammett plot equals -2.2. Variation of the nature of the alkyne results in smaller and less systematic changes in the rate constants. The volume of activation ranges from-18.6 to-14.4 cm3mol-’ in the case of P h C 4 P h but is practically zero in the case of E t C d E t . The monomerization of dimers 1 by either pyridine or [NEt4]Cl-H20 to afford the species [Pd(CsH3RCHzNMe2)Cl(L)](L = py or C1, charges are omitted) retards the insertion markedly, suggesting that only the dimers are the reactive species and that a halo bridge is essential for the entrance of alkyne into the coordination plane of Pd(I1). The insertion of the second alkyne was kinetically studied by reacting the seven-memberedcomplex 4 with P h C d P h . Although this step is associated with a cis trans rearrangement about the C==C bond of 4, as shown by X-ray crystallography, its formal kinetic characteristics are very similar to those of the first insertion step. Both the steps are driven by the nucleophilic attack of the phenyl or alkenyl carbon on the coordinated alkyne. A more complicated mechanism of the second insertion, which is essential to account for the cis -trans isomerization, is discussed in some detail.
-
Introduction Insertion of alkynes into Pd-C bonds of palladacycles is one of the representative processes that makes cyclopalladated compounds attractive starting materials in organic synthe~is.~*~ In fact, much has been done in terms of synthesis of various, often unique organic and organometallic compounds starting with palladacycles and alkynes since the first report in 1979.4 All these transformations have the same key steps, viz. the insertion of one or two alkynes into the Pd-C bond to yield complexes of type 2 or 3, respectively, depending on the nature of the alkyne.3 Until now, however, these transformations have (1) Alexander von Humboldt Fellow 1991-1993 a t the Institut filr Anorganische Chemie, UniversitAt WittdHerdecke, Stockumer Str 10,
_ _ _ _ ______
5810 Witten (GermanvL .. .-__I_. ~, .
(2) Ryabov, A. D. Synthesis 1986, 233. (3) Pfeffer, M. Reel. Trau. Chim. Pays-Bas 1990, 109, 567. (4) Baheoun, A.; Dehand, J.; Pfeffer, M.;Zinsius, M. J. Chem. Soc., Dalton Tram. 1979, 547.
not been studied kinetically, although the mechanisms have been discussed on the basis of general chemical evidencee6Therefore, we report in this paper a detailed high-pressurekinetic study of the insertion of alkynes into the Pd-C bonds of ring-substituted N,N-dimethylbenzylamine complexes 1 to afford doubly inserted species 3, and of the insertion of diphenylethyne into the Pd-C bond of the “singly inserted” complex 4 to yield the “doubly inserted” compound Sa. The mechanistic conclusions concerning the latter interconversionare supported by an X-ray crystallographic study of the similarly prepared model compound Sb. The results obtained are compared with the correspondingkinetic data on reactions of alkenes with palladacycles of type 1 which have previously been rep~rted.~J (5) Arlen, C.;Pfeffer, M.; Bars,0.; Grandjean,D.J. Chem.Soc.,Dalton Tram. 1983,1535. (6) Ryakv, A. D.; Sakodinskaya, I. K.; Yatsimiraky, A. K. J.Chem. SOC.,Perktn Tram. 2 1983, 1511.
0276-7333/93/2312-1386$04.00/00 1993 American Chemical Society
Organometallics, Vol. 12, No.4, 1993 1387
Alkyne Insertion into Palladacycle Pd-C Bonds
Table I. Summary of Crystal Data and Intensity Collection of Sb
Chart I r
1
R
*+Q-
C24H2&lNO4Pd*O.SC7Ha triclinic
a, A
10.91(1) 14.21 (1) 8.95 (1) 106.01 (5) 92.34 (5) 99.78 (5) 1307.7 580 2 0.55 X 0.45 X 0.14 0.710 73 8.3 594 1.00 0.35 tan 0 1-25 4921 4117 ( I > 3 4 0 ) 0.026 0.037 1.33
b, A
c, A a,deg
2
1
chemical formula cryst syst space group
b, deg
2
cryst dimens, mm
a(MoKa),A
w, cm-1 F(ow
scan range, deg 0 limits, deg no. of reflctns, total no. of reflctns, used R factor, % R, factor, % std error in an observn of unit weight, e
4
5
R=Ph(a),Me@)
Experimental Section Organic Molecules. Diphenylethyne and hex-3-yne were obtained from Aldrich. Phenyl(4nitrophenyl)ethyne and phenyl(3-trifluoropheny1)ethynewere synthesized as described previously.s (Dimethy1amino)methylbenzeneand 1-[(dimethylamino)methyl]-3-methoxybenzenewere purchased from Aldrich. Chloroform stabilized with 60 ppm 2-methyld-butene (Merck) was used as solvent throughout this study unless otherwise indicated. Pyridine (Merck) was distilled and tetraethylammonium chloridemonohydrate (Merck)was kept at 120"C overnight before use. 1-[(Dimet hy1amino)met hyll-4-fluorobenzene. 4-Fluorobenzoic acid (24 g, 0.17 mmol) was refluxed in 25 mL of SOClz until all product had dissolved, yielding a pale yellow solution (ca. 15 min). An excess of SOClz was distilled off under reduced pressure followed by distillation of the resulting acid chloride. After drying in vacuo, the latter was transferred into a dropping funnel and added slowly to aqueous HNMe2 (40%,100mL). The pH of the resulting solution was adjusted to ca. 1by concentrated HC1, and the solution was evaporated to dryness. The chlorohydrate salt of the amide was dissolved in a minimum amount of H20, and solid KOH was added to make the pH ca. 14. The product was extracted with 3 X 200 mL of EhO. After drying by MgS04 and purification with activated carbon, the solvent was evaporated to yield, after drying in vacuo, 25 g (88%)of the pure amide. This product was disaolved in 150 mL of dry EhO, and the solution was added slowly to a cooled, clear solution of 14 g of LiAlH4 in 150 mL of freshly distilled dry EtzO. After addition (1.5 h, the temperature should not rise above 10 "C) the mixture was refluxed for 4 h. The excess of LiAE& was carefully destroyed with EtOAc followed by HzO and finally with 15% aqueous HCl. The milky reaction mixture was left overnight and extracted with EhO (3 X 200 mL). After drying with MgSOr the solvent was evaporated and the resulting yellow oil distilled at reduced pressure to give 11g (50%) of the product. lH NMR (6, CDCl3): 2.22 (a, 6H, CH3), 3.38 (8, 2H, CHz), 7.00 and 7.23 (2m, 4H, Ar). Palladium Complexes. Dimers la and IC were prepared (7) Ryabov,A. D.;Sakodinskaya,I. K.; Yataimiraky, A. K. J. Organomet. Chem. 1991,406,309. (8) Beydoun, N.; Pfeffer, M. Synthesis 1990,729,
Pi (No.2)
+
accordingto ref 9 exceptthey were additionallypurified by column chromatography (Si02-CHCl3); complexes l b and Id were synthesized according to a general procedurelo in 87 and 91% yields, respectively. l b lH NMFt (CDCl, + py-ds) 2.07 (8, CH3), 2.91 (s,NCH3), 3.93 (s,NCH2),5.79 (a, H6),6.83 (m,H3 + H4). Anal. Calcd for C&&lzNzPdz: C, 41.40; H, 4.86; N, 4.83. Found C, 41.10; H, 4.81; N, 4.84. I d 'H NMR (CDCla + py-ds) 2.90and 2.91(2s,NCH3),3.95(s,NCH2),6.65 (m,H6),6.71and 6.93 (2m, H3 H4). Anal. Calcd for CleHnCl$ZNzPdz: C, 36.76; H, 3.77; N, 4.76. Found: C, 36.61; H, 3.66; N, 4.87. Iodo-bridged dimer le was prepared as described in ref 11. Complexes 3c and 3b (R'I'' = Ph) were prepared as described in refs 4 and 12, respectively. The other compounds 3 were obtained analytically pure by identical procedures starting from the corresponding cyclopalladated compounds 1. The syntheses of 4 and Sa,b are described elsewhere." Compound 6 was obtained from la and py.9 'H NMR (CDCl3): 2.93 (8, NCH,), 3.71 (8, OCHd, 3.94 (8, NCHz), 5.89 (d, J 8 Hz, H6), 6.38 (dd, J 8,3 Hz, H5), 6.61 (d, J 3 Hz,H3),7.36 (m,H3',5'),7.81 (m,H4'),8.86 (m, H2',6'). Anal. Calcd for Cl&l&lNzOPd: C, 47.14; HI 5.01. Found C, 46.77; H, 4.97. Kinetic and Other Measurements. The reactions at ambient conditions were studied spectrophotometrically on a Shimadzu UV-250 spectrophotometer equipped with a temperature-controlled (fO.l "C) cell compartment providing three independent measurements. The reactions at pressures up to 150 MPa were studied on a Zeiss PMQII spectrophotometer equipped with a therlrioetated (hO.1 "C) high-pressure cell13by employing the pill-box technique.14 The kinetic traces were analyzed by using a homemade computer program designed by A. Neubrand. 'H NMR spectra were run on an AM 400 WB Bruker instrument. X-ray Diffraction. Intensities were measured on an EnrafNonius CAD-4 diffractometer. The crystal parameters and a summary of data collection and structure refinement are given in Table I. No intensity decay was observed during the data collection period. Corrections for the Lorentz and polarization
+
(9) Ryabov, A. D.; Polyakov, V. A.; Yataimirsky, A. K. J. Chem. SOC., Perkin Trans. 2 1983,1503. (10) Pfeffer, M. In Znorg. Synthecris; Kaesz, H. D., Ed.; WileyInterscience Publishers; New York, 1989; Vol. 26, p 211. (11) Maasearani, F.; Pfeffer, M.; Le Borgne, G. Organometallics 1987, 6, 2029. (12) Pfeffer, M.; Sutter, J. P.; Rotteveel, M. A.; De Cian, A., Fischer, J. Tetrahedron 1992,48,2427. (13) Fleiechmann, F. K.; Come, E. G.; Stranks, D. R.; Kelm, H. Rev. Sci. Imtrum. 1974,46,1421. (14) Le Noble, W. J.; Schlott, R. Rev. Sci. Znstrum. 1976,47, 770.
Ryabou et al.
1388 Organometallics, Vol. 12, No. 4, 1993 effect were applied, but not for absorption owing to the low value of the linear absorption coefficient. The structures were solved by the Enraf-Nonius SDP package on a PDP 11-60 computer.15 The atomic positions of the independent non-hydrogen atoms were found with the program MULTAN and the subsequent Fourier difference synthesis.After refinement of coordinates and isotropic thermal parameters of the 31 independent atoms of the molecule, peaks corresponding to a molecule of toluene were found on a Fourier difference, near an inversion center. Three carbon atoms (C25, C26, and C27) with an occupancy factor of 0.5 constitute the independent atoms of one-half molecule of toluene per asymmetric unit. Refinement of coordinates and thermal parameters, first isotropic and then anisotropic, of the 35 independent non-hydrogen atoms with fixed coordinates and thermal parameters led to the final R values reported in Table I.
Results General Observations and Rate Law. The reactions of two alkyne molecules of the type R’CsCR” with the Pd-C bonds of the orthopalladated N,N-dimethylbenzylamines 1to form 3 occur under syntheticconditions in high yields.4 The same is true when these are run under the kinetic conditions, i.e. when a large excess of alkyne, [alkyne] >> Ell, is employed. Under these conditions, the yields of 3 exceed 90% and the reaction progress can easily be followed spectrophotometrically especially in the case of ArC=CAr’ alkynes. In fact, complex IC has an absorption maximum a t 340 nm (E = 1300 M-l cm-l, with respect to one Pd unit), whereas 3c (R’/” = Ph) that possesses a system of conjugated C = C bonds and phenyl rings has a shoulder at 400 nm (E = 4000 M-’ cml). Since incoming alkynes do not usually absorb at 400 nm, the reactions were followed at this wavelength in the majority of cases. The exceptionsare the reactions between ICand EtCd2Et (310 nm), ICand PhMC6H4N02-4 (435nm), and 4 and P h C W P h (410 nm). Good pseudo-first-order behavior is a typical feature of all reactions studied, and this was the case for at least 4-5 half-lives. Observed pseudo-first-order rate constants, k(obsd), derived from these traces were independent of the concentration of complexes 1 and depended linearly on the concentration of the alkyne, as shown in Figure 1. The appropriate rate law is given in (1). The values of the k(obsd) = k1[R’C=CR’’1
(1)
rate constants kl along with the corresponding activation parameters AH* and AS* obtained from the values of kl measured at 25.0, 35.0, and 44.1 “C are summarized in Tables I1 and 111. The data in Figure 1indicate a significant dependence of kl on the substituent R in the aryl ring of 1. The corresponding Hammett plot is shown in Figure 2, and the analytical form of this line is given in (2). The relatively log kl = (-1.24 f 0.05) - (2.2 f 0 . 2 ) ~
(2)
large value of the slope for Pd(I1) chemistry suggests the importance of the electronic factors and, in particular, of the nucleophilicity of the aryl carbon C1. In contrast, electronic effects in alkyne molecules are much less pronounced and rather unsystematic, as demonstrated by the data in Table 111. (15)Frenz, B. A. In Computingin Crystallography;Schenk, H., OlthofHazelkamp, R., van Konigsveld, H., Bassi, G. C., Eds.; Deltt University Press: Holland, 1978.
104xk(obsd), s-l 20
15
10
5
[I
0
2
4
6
8
10
12
14
16
IS
20
22
103x[PhC=CPh], M Figure 1. Pseudo-first-orderrate constantsfor the reactions of the ring-substituted chloro-bridged complexes 1 with diphenylethyne as a function of the concentration of PhCrCPh. Conditions: chloroform solvent; 25 O C .
Pressure Effects. The insertion of PhCd2Ph was accelerated significantly by pressure in the range 10-150 MPa in all the cases studied, Figure 3. Correspondingly, the negative activation volumes, AV*,were calculated and these are in the range -18.6 to -14.4 cm3mol-’, Table 11. The rate of thereaction of the lessbulky alkyne,E t W E t , with ICwas, on the contrary, unaffected by pressure, and the value of AV* is practically zero, see Table 111. Reaction of Diphenylethyne with 4. The synthesis of 5a by the treatment of 4 with 1equiv of PhC=CPh was shown to occur in a more than 60% yield.” We have now checked that under kinetic conditions, i.e. in excess of 20 equiv of diphenylethyne with respect to 4, the same reaction is observed, affording compound Sa in a quantitative yield, as far as the sensitivity of lH NMR is concerned. In contrast to this result, we have recently found that the reaction of compounds analogous to 4, but with substituents in the aryl ring (5-F or 4,5-OCHz0), did not lead to the expected compounds of type 5 but rather of type 3; i.e. the hex-3-yne residue is substituted by a diphenylethyne molecule. This behavior is however beyond the scope of this work and will be analyzed in detail elsewhere. To this end, we have followed the kinetics of the clean conversion of 4 into Sa and the structure of the product has been proven by an X-ray crystallographic study of the very similar compound 5b (see below). From the kinetic point of view, the reaction between 4 and diphenylethyne is very similar to the corresponding reactions of complexes 1in terms of the rate law, the values of kl, AH*,ASs, and AVs, Table 11. It should also be pointed out that during the synthesis of 5a, which was followed by lH NMR, we found no evidence for the formation of any kind of intermediate, the only signals that were identifiable being those of either 4 or 5a. X-ray Crystal Structure of 5b. Several crystal structure determinations have been performed on compounds deriving from insertion of two alkynes into the Pd-C bond of cyclopalladated c o m p l e ~ e s . ~All J ~these structures have a common feature in that they all display (16) Ricci, J. S.;Ibers,J.A.J. Organomet.Chem. 1971,27,261. Dupont, J.; Pfeffer, M.; Daran, J. C.; Gouteron, J. J . Chem. Soc., Dalton Trans. 1988,2421. Tao, W.; Silverberg, L. J.; Heck, R. F. Organometallics 1989, 8,2550. Albert, J.; Granell, J.; Sales, J. J. Organomet. Chem. 1989,379, 177. Pfeffer, M.; Rotteveel, M. A.; Sutter, J. P.; De Cian, A.; Fischer, J. J. Organomet. Chem. 1989,371, C21. Sutter, J. P.; Pfeffer, M.; De Cian, A.; Fischer, J. Organometallics 1992, 11, 386.
Alkyne Insertion into Palladacycle Pd-C Bonds
Organometallics, Vol. 12, No. 4, 1993 1389
Table 11. Second-Order Rate Constants 4 and Activation Parameters for the Insertion of P h m P h into the P d - C Bond of Cyclopalladated Complexes in Chloroform Complex
(R,X)
T/OC
k,/M-l s-'
25 35 44.1 35 25 35 44.1 25 25 35 44.1 25 35 44.1 25 35 44.1 25 35 44.1
(0.75 f 0.04) X 1W2 (2.49 f 0.08) X 1W2 (5.46 f 0.14) X 1F2 (1.63 f 0.03) X 1W2") (7.1 f 0.6) X 0.146 f 0.003 0.253 f 0.017 (6.4 f 0.9) X 10-2 a (4.9 f 0.4) X (9.4 f 0.5) X 0.20 f 0.02 0.102 f 0.007 0.218 f 0.013 0.363 f 0.013 0.196 f 0.027 0.30 f 0.04 0.53 f 0.05 (1.48 f 0.12) X (3.66 f 0.23) X 1W2 (7.96 f 0.23) X
AH+/kJ mol-'
hs#/JK-'mol-'
AVf/cm3 mol-'
71 f 1
-47f5
-18.6 f 0.9
50f2
-99 f 6
-15.0 f 0.5
56f5
-82 f 17
50 f 4
-97 f 12
50.1 f 0.7
-93 f 2
-16.2 f 1.4
70.1 f 2.1
-42.9 f 6.9
-14.4 f 1.2
HOAc as solvent.
Table III. Second-Order Rate Constants kland Activation Parameters for the Insertion of'"R ' into the P d - C Bond of Complex ICin Chloroform alkyne R'/R'' Ph/Ph PhfC6H4CF3-3 PhfCbHdN02-4 EtfEt"
a
T/OC 25 25 35 44.1 25 25 35 44.1
k1fM-l
SS'
(7.1 f 0.6) X (3.2 f 0.2) X (6.8 f 0.3) X 1W2 0.144 f 0.005 (4.9 f 0.4) X (7.46 f 0.06) X 0.14f0.01 0.32 f 0.04
AHffkJ mol-!
K-I mol-]
50 f 2
-99 f 6
59 f 2
-76 f 8
57f5
-76f 18
ASffJ
A P = 0.9 f 0.7 cm3 mol-]. 0
log kl (t4-ls-I) I
20
40
60
80
100
120
140
160
Pressure, MPa Figure 3. Effect of pressure on the second-order rate constants 121 for reactions of the ring-substituted chlorobridged complexes la (MI, IC(W, 4 (dvo), and Id (A)with diphenylethyne and complex I C ( 0 ) with hex-3-yne. Conditions: chloroform solvent; 25 O C .
-0,2
0
092
0,4
Hammett sigma constant Figure 2. Hammett plot for the second-order rate constants kl(25 "C) versus the u constantsof substituentsin the phenyl ring of chloro-bridged complexes 1. the same trans arrangement about the alkene C 1 ' 4 2 ' bond. All these compounds, however, were directly obtained from either diphenylethyne or hex-3-yne; i.e. in none of their syntheseswas the singlyinserted intermediate isolated, and therefore there was no evidence that the latter had indeed a cis arrangement of the palladated Cl'=C2' alkenyl unit. We have therefore performed an X-ray diffraction study on a single crystal of Fib which has indeed been synthesized in a stepwise fashion from complex 4 and 1-phenylpropyne. The result of this study is given in
Figure 4 and in Tables IV-VI. The molecule 5b consish of a butadienyl chain #;q2-bonded to the Pd atom, the third and the fourth coordination sites of the latter being occupied by the nitrogen and a chloro ion, respectively. The distances and the angles are within the expected range;4J6however, the most important information is that the two methoxycarbonyl groups are mutually trans in the alkenyl unit v2-bonded to the palladium center. This result is thus a definite proof that the insertion of the second alkyne into the Pd-C bond and the isomerization of the alkenyl unit in 4 are taking place simultaneously. Rsactivity of Monomeric Palladacycles. From a mechanistic point of view it is of importance to compare the reactivity of the dimeric complexes 1 and their monomeric counterparts derived from 1 in the presence of either pyridine" or chlorals ligands according to eq 3. When isolated complex 6 was introduced into the reaction (17) Dehand, J.; Pfeffer, M. Coord. Chem. Reu. 1976, 18,327. Bruce, M. I. Angew. Chem., Int. Ed. Engl. 1977,16, 73. Omae, I. Chem. Reu. 1979,79,287. Coord. Chem.Reu. 1979,28,97; 1980,32,235;1982,42,245; 1988, 83, 137. Idem. Organometallic Intramolecular-coordination Compounds; Elsevier Science Publishers: Amsterdam,New York, 1986.
['5.8"5
Ryabov et al.
1390 Organometallics, Vol. 12, No. 4, 1993
Table V. Selected Bond Length (A) with Esd's in Parentfor Compound 5b
c9c
Pd-CI Pd-N Pd-C 1 Pd-C3 Pd-C4 Cl-C2 C2-C3 c3-c4
c10
2.310(1) 2.209(2) 2.007(2) 2.177(2) 2.17 l(2) 1.326(3) 1.490(3) 1.392(3)
CM5 C5-C6 Cbcll Cll-N c344 CM16 N-Cl2 N-Cl3
1.505(3) 1.397(3) 1.499(3) 1.481(3) 1.392(3) 1.521(3) 1.475(3) 1.474(3)
Table VI. Selected Bond Angles (deg) with Esd's in Parentheses for Compound Sb CI-Pd-N Cl-Pd-C1 N-Pd-C 1 N-Pd-C3 N-Pd-C4 C 1-Pd-C3 C 1-Pd-C4
91.08(5) 93.14(6) 172.90(7) 109.32(7) 92.8 l(7) 65.08(8) 84.95(8)
N-C11-C6 Cll-C6-C5 Cbc5-C4 c5-C4-C3 C4-C3-C2 C342-Cl C3-C4-C16
116.0(2) 125.1(2) 125.2(2) 125.1(2) 119.0(2) 106.3(2) 117.4(2)
L.) c 18 c23(J%
Figure 4. ORTEP view of complex Sb. Table IV. Final Positional and Equivalent Isotropic Thermal Parameters (A*)with Esd's in Parentheses for the Non-Hydrogen Atoms of Compound 5b atom Pd
c1 N c1 c2 c3 c4 01 02 03 04 c5 C6 c7 C8 c9 c10 c11 c12 C13 C14 C15 C16 C17 C18 C19 c20 c21 c22 C23 C24 C25 C26 C27 C28
X
Y
z
B O
0.08773(1) -0.07224(5) 0.1667(2) 0.016ii2j 0.0728(2) 0.1667(2) 0.2604(2) 0.1212( 3) 0.2176(2) 0.2969(2) 0.3079(2) 0.373l(2) 0.3713(2) 0.4860(2) 0.5977(2) 0.5991(2) 0.4880(2) 0.2556(2) 0.0665(3) 0.2287(3) 0.1666(2) 0.2088(4) 0.2872(2) 0.3408(4) 0.0578(3) -0.0803(2) -0.0691 (2) -0.1554( 3) -0.2562(3) -0.2695(3) -0.1826(2) 0.4287(4) 0.5147(4) 0.5904(4) 0.5390(9)
0.28883(1) 0.37435(5) 0.3267(2) 0.2367i2j 0.1596(2) 0.1667(2) 0.2513(1) 0.0572(2) 0.0116(1) 0.2832( 1) 0.41 19(1) 0.2623( 1) 0.271 5(2) 0.2865(2) 0.2918(2) 0.2801(2) 0.2665(2) 0.26 17(2) 0.3 108(3) 0.4324(2) 0.0745(2) -0.0893(2) 0.3 153(2) 0.4794(2) 0.0806(2) 0.2690(2) 0.3686(2) 0.3995(2) 0.3323(2) 0.2331(2) 0.2018(2) 0.0462(3) -0.0041(3) -0.0525(3) -0.0069(7)
0.1 5994(2) 0.14388(7) -0,0441(2) 0.3311(2) 0.3171(3) 0.2024(2) 0.2349(2) -0.0607(2) 0.1246(3) 0.5126(2) 0.4140(2) 0.1452(2) -0.0062(3) -0.0696(3) 0.01 23(3) 0.1 589(3) 0.2254(3) -0.1 101(3) -0.1693(3) 0.0006(3) 0.0691(3) 0.0235(5) 0.4038(2) 0.5689(4) 0.4005(3) 0.4308(2) 0.5159(3) 0.6159(3) 0.6325(3) 0.5498(3) 0.4484(3) 0.4286(6) 0.3463(6) 0.4220(6) 0.199( 1)
2.416(3) 4.08( 1) 3.62(4) 2.80(4) 3.00(4) 2.74(4) 2.42(4) 5.96(6) 5.76(5) 4.26(4) 3.80(4) 2.51(4) 3.04(4) 4.00(5) 4.26(6) 3.77(6) 3.13(4) 3.62(5) 6.38(8) 5.26(6) 3.44(5) 7.6(1) 2.72(4) 5.47(7) 4.37(6) 2.85(4) 3.42(5) 4.35(6) 5.08(7) 5.06(7) 4.08(6) 7.1(1) 8.0(1) 0.2(1) 8.8(3)
Anisotropically refined atoms are given in the form of the isotropic equivalent displacementparameters defined as (4/3) [a2j3(1,l) + b2j3(2,2) + c2j3(3,3) + ab(cos y)j3(1,2) + ac(cos j3)@(1,3)+ bc(cos a)j3(2,3).
mixture and reacted with P h C W P h instead of the parent dimer la, the value of kl was (6.4f 0.1) X 103 M-I s-l at (18) Braunstein, P.; Dehand, J.; Pfeffer, M. Znorg. Nucl. Chem. Lett.
1974, IO, 581.
25 "C in CHCl3; i.e. the reaction rate decreased by a factor of 31. In the presence of 0.004 M py the rate decreased further and the second-order rate constant kl appeared to be equal to (3.9 0.1) X 10-4 M-l s-l, as was estimated from the initial rate of formation of 3a, since the rate was too slow to use the integral method. The overall rate retardation on going from dimer la to monomer 6, stabilized against the reverse dimerization by 0.004M py, appeared to be a factor of 500. This effect demonstrates the negligible reactivity of the "py" monomer as compared to the parent dimer la. The effect of monomerization of la by [NEt4lCl*H20 was investigated in more detail by generating the monomeric species 7 in situ. A plot of kl as a function of concentration of the salt is shown in Figure 5. As in the py case, there is a strong rate retardation. In order to prove that the effect is due to conversion of la into 7, the interaction between the dimer and [NEt~ICl-H20was studied spectrophotometricallyin CHC13at 25.0,35.0,and 44.1 "C. Dimer la has a maximum at 341 nm (c = 1435 M-l cm-l). On addition of the salt the absorbance decreases slightly,the maximum moves to 339 nm, and the isosbestic point at 307 nm is observed. The equilibrium is reached much faster as compared to the insertion reaction. The dependence of absorbance on the concentration of [NEt41CbH20 is demonstrated in Figure 6. The two profiles in Figures 5 and 6 are very similar, indicating that the one and the same phenomenon, namely the dimer e monomer equilibrium (3)must account for both. From the equilibrium measurements, using the approach described in detail elsewhere,19the equilibrium constant K3 of 95.1 f 1.9 M-l(25 "C) was calculated for the chloro case and the values of K3 showed a minor temperature dependence. Using this value of K3, the equilibrium concentrations of dimer la in the presence of [NE&JCl-H20 were calculated and the values of kl are plotted against
*
(19) Ryabov, A. D. Znorg. Chem. 1987,26, 1252.
Alkyne Insertion into Palladacycle Pd-C Bonds
Organometallics, Vol. 12, No. 4, 1993 1391
102xk1, t 4 - l s - l
2ol t
10-
t 5t t
o-“““’“~”“’‘“”
(
.
~
t ‘
~
‘
.
.
(
‘
‘
‘
l
~
~
,
l
.
’
~
l
’
~
.
‘
Reactions in Acetic Acid as Solvent. Styrenes react with the Pd-C bonds of orthometalated N,N-dimethylbenzylamines showing a strong acid ~atalysis.6.~The reactions do not occur in CHC13, and acids, HOAc or HC104, are required to trigger the process. It was thus interesting to compare the reactivity of alkenes and alkynes with respect to 1 from the point of view of the acid catalysis. The reactions of diphenylethynewith ICand Id were run in acetic acid as solvent. In both cases the reactions proceed with measurable rates and the rate constants kl are included in Table 11. As seen, the values of kl for HOAc are only slightly lower as compared to CHC13 as solvent. Thus, there is no acid catalysis in reactions of alkyneswith orthopalladated N,N-dimethylbenzylamines. Discussion Formal Kinetics. The simple rate law (1)and the firstorder dependence in the concentration of alkyne, in particular, support the “synthetic” evidence“ for the fact that the insertion of the first alkyne molecule is the ratedetermining step. In fact, when a 1:l mixture of IC and P h C N P h was reacted, the products were still complex 3c (R’l’’ = Ph) and nonreacted IC;the formation of 2 was never observed.ll From the kinetic standpoint, one may write the following formal sequence:
o,8p t
t
0171
0,65
,
0
I
*
0,751
,
/
2
/ , ,
,
2
+
1 R’CsCR’’
4
6
,,;, , , , , , ,
8
10
12
~
, , , , ,
14
, , , , / , , ,
16
18
~
20
,
,
,
,
22
103x [ N E t 4 C l I XHpO, M
Figure 6. Change in absorbance of solutions of complex la in CHC13 on consecutive addition of [NEtd]Cl.H20 at 25 O C and [Pd(II)]t 5.56 X 1V M. 102xk1, M - l S - l
~ i2:
+ R’CsCR’’
-
k4,k,
(4)
k,
(5)
3
Application of the steady-state approximation with respect to the concentration of 2 results in the expression for k(obsd) given in eq 6. The first-order dependence in [R’CECR’’] only holds on the condition that kg[R’CMR’’] >> k-4. If so, eq 6transforms into eq 7, which is in agreementwith the experimental rate law, for which kl = k4. Another source of evidence for the slower first k(obsd) =
k,k, [R’C”‘’] k, + k,[R’C=CR’’I
k(obsd) = k,[R’”CR’’l
105x[la], M
Figure 7. Rate constants kl for the reaction of P h C 4 P h with la in the presenceof [NEXJCEH20 from Figure 5plotted 88 a function of the calculated concentration of the dimeric speciesfrom the equilibrium constant K3estimated from the data in Figure 6. these in Figure 7. A satisfactory correlation is evident and supports a decisive role of the dimeric species in the reaction.
(6)
(7)
insertion reaction may be obtained from the comparison of the rate constants kl for the ring-substituted complexes 1 and complex 4. From the Hammett equation (2) it is possible to estimate the rate constant k l for the insertion of P h C N P h into the complex of type 1with two MeOOC substituents and to compare it with the measured rate constant for the second insertion of P h C N P h into 4, since this complex contains also two electron-withdrawing MeOOC groups. Naturally, this comparison assumes a close sensitivity of the fiit and the second insertion toward electronic effects. Using the Hammett value for the MeOOC group of 0.4,the calculated value equals 0.1 X le2M-ls-l and must be compared with the measured one of 1.48 X le2M-l s-1 at 25 OC. The former is in fact by a factor of 15 lower. What Is kd? One of the key questions related to the mechanisms of insertion of unsaturated molecules into Pd-C bonds of dimeric halo- or acetato-bridged palladacycles is how the incomingmolecule enters the coordination sphere of Pd(I1)for the subsequentinsertion into the Pd-C bond.697 There are two possibilities in the present case, i.e. the “bridge-opening” entrance via intermediate 8 and
Ryabov et al.
1392 Organometallics, Vol. 12, No. 4, 1993 Scheme I
7 7
a-W-N
N-4-6
N-&b
I
8
Q
10
1
N - 4 - u
+
111
K
47 sp-N
u
C L
the axial route via 9, where the latter is actually indistinguishable from the direct insertion of alkyne from the bulk solvent. Since alkynes react readily in aprotic chloroform and the reactions are not subjected to acid catalysis, the entrance via Pd-N bond breaking7 can be excluded from the present considerations. Negligible reactivity of the monomeric species 6 and 7 as compared to the parent dimer, is strong evidence in favor of the bridge-openingentrance via 8. Conversion of the dimers into 7, in particular, must increase the nucleophilicityof the complex and hence, as suggested by the Hammett plot in Figure 2, enhance the reactivity. However, the opposite effect is observed,indicative of the primary importance of the bridging structure. Bridging ligands, in principle, are always a potential site of coordinative unsaturation, since a cleavage of one bridge immediately creates a coordinative vacancy at the metal center. Another way to enforce coordinationunsaturation and, thus, to initiate the reactions of alkynes with low reactive palladacycles is to abstract the chloro ligands by AgBF4 to produce solvent0 species.l1Yz0 From the point of view of the bridging entrance, the reactions of alkenes7 and alkynes are similar: this seems to be a dominant path. “Synthetic”evidence for the importanceof this route comes from the C-C bond formation that is occurring in the reaction between metallacarbynes and cyclopalladated compounds. We have indeed shown that metallacarbynes do behave like organic alkynes and the M* bond undergoes formal insertion into Pd-C bonds. An interesting feature of this reaction is that we could demonstrate that prior to the insertion, an intermediate compound having the metallacarbyne unit coordinated to Pd(I1) is formed.21 The stoichiometric mechanism of the first insertion may be written as shown in Scheme I. The equilibrium driven by K must be shifted strongly to the left, since no intermediates were detected spectrophotometrically. Therefore, the rate constant k4 (or kl) is given by k4 = Kkim, and hence, the substituent effect and all activation parameters are the s u m of terms arising from K and kins. Thus, these parameters must be considered separately. Substituent Effect. The slope of the Hammett plot of -2.2 is rather negativezzand must be compared to the p values of -1.13 and -1.57 for reactions of styrene with the Pd-C bonds of 1 and orthopalladated acetanilide, re~pectively.~ This implies that the negative charge at the C1 carbon plays an important role in the reaction. Unfortunately, it is impossible to study the substituent effects on K and kin8 separately, but some literature comparisons, namely an equation of type (3) for 2phenylpyridine,ls suggest that a slightly positive value of
to expect that the insertion step itselfwillnot be associated with a large change in such that AV*(obsd) will mainly represent AVO(K) for the addition process. This value should correlate with the bulkiness of the alkynes, since the larger these are the more effective will the overlap of the molecularspheres be, i.e. a more negative reaction volume. This is indeed the case when the data for the insertion of PhC=CPh and E t C d E t are compared. The precoordination of the incoming alkynes via the bridge-opening process can be visualized as an aasociative process that is accompaniedby a significantvolume collapse. By way of comparison, AV* values between -17 and -25 cm3mol-1 have been reported for the insertion of electron-rich triple-bonded systems into metal-carbene bonds of pentawbonyl(methoxypheny1carbene)chromium and -tungsten complexes.26 Similarly, the [2 + 21 cycloaddition reactions on the coordinated ligand of chromium and tungsten pentacarbonyl carbene complexesare characterized by significantly negative volumes of activation, viz. between -15 and -18 cm3mol-1.26 It follows that the insertion of alkynes is in line with an associative
(20) Wu, G.;Rheingold, A. L.; Heck, R. F. Organometallics 1987, 6, 2386. (21) Engel, P. F.; Pfeffer, M.; Fischer, J.; Dedieu, A. J . Chem. Soc., Chem. Commun. 1991, 1275. (22) For other “Hammett” extremes in Pd(I1) chemistry see: Yatsimirsky,A. K.; Deiko, S.A,; Ryabov, A. D. Tetrahedron 1983,39,2381 ( p = -3.0). Reference 19 ( p = -2.93).
(23) van Eldik, R.; Asano, T.;le Noble, W. J. Chem. Reu. 1989,89,549. (24) van Eldik, R.; Merbach, A. E. Comments Inorg. Chem. 1992,12, 341. (25)Schneider, K. J.; Neubrand, A.; van Eldik, R.; Fiacher, H. Organometallics 1992,11,267. (26) Pipoh, R.; van Eldik, R.; Wang, S. L. B.; Wulff, W. D. Organometallics 1992, 11, 490.
klns
#
)CI -
a-Pd-N
N-&&
I
/
I
i D
fast
PRODUCTS
p(K)
is to be expected. Since
p(obsd) = P ( K )+ ~(ki,,) (8) the value of p ( k h ) may be more negative than -2.2. This means that the transition state for the first insertion is to a large extent nonconcerted and Cl-Cl’ bond making plays a major role in the transition state. Pressure Effects. In terms of the suggested mechanism in Scheme I the observed volume of activation will be given by the s u m of the reaction volume for the formation of the adduct and the activation volume for the rate-determining insertion step, eq 9. It seems reasonable AV’(obsd) = A P ( K )
+ AVS(kh)
(9)
Alkyne Insertion into Palladacycle Pd-C Bonds process during which the volume collapse is mostly due to the precoordination of the incoming alkynes via the bridge-opening reaction. The associative nature is most probably the reason why chloro and iodo complexes IC and If, respectively, react with similar rates, since it is well decumentedthat associativereactions have a reduced sensitivity toward the nature of the leaving ~ ~ O U P S . ~ The mechanism in Scheme I is very similar to that proposed for insertion of alkeneseJ into the Pd-C bonds of complexes 1, but the reaction rates are markedly different. Alkynes react much faster, and the difference is really enormous in nonprotic solvents. The possible reason is that an v2-coordinated alkyne in 8 type intermediates has to a considerable extent a contribution from the resonance structure of type 10. This hypothesis is in line with that proposed recently by Sylvestre and Hoffmann who analyzed the v2-alkyne-vinylidene rearrangement occurring in T-shaped ds MLs(alkyne) complexes.28 Therefore, alkynes should attain enhanced reactivity toward nucleophiles on coordination to Pd(I1). Speculations about the Insertion of the Second Alkyne. The conversion of 2 into 3 cannot be probed kinetically. However, this is possible for the formation of 5a from 4. Since 2 and 4 are both halo-bridged dimers, it is reasonableto assume that the second insertion match@ the first one, i.e. the bridge-opening entrance is followed by the nucleophilic attack of the C2’ atom at the alkyne. However, the cis to trans isomerization at the C==Cbond is still poorly understood. One might speculate that a reasonable reaction sequencemight be as shown in Scheme 11. The key intermediate here might be complex 11 formed after nucleophilic attack of the alkenyl C2’ carbon u-bonded to Pd at the positively charged carbon of coordinated alkyne. It may appear that the Pd-C2‘bond is not broken after the new C-C bond formation, and this would lead to the palladacyclobutenyl intermediate 11. The latter may undergo the Pd-C2’ bond cleavage accompanied by the reformation of the C l ’ 4 2 ’ double bond. Manipulationswith the molecular models support the idea that this reformation can easily bring about the trans arrangement of R groups at the Cl’=C2’ double bond. The isomerization is likely of a “thermodynamic” origin, since, as suggested again by the molecular models, only the trans-configurated +coordinated alkenyl fragment Cl’=C2’ will be perpendicular to the palladium plane, if the Pd-N bond is still present. For the cisconfigurated fragment,one must expect the C1’42’bond to be in the Pd plane, and this is, of course, thermodynamically less favorable. The mechanism in Scheme I1 is different as compared to that proposed previously for the cis trans iaomerization which was based upon a metallacyclic flip that might occur after the insertion of the second alkyne has taken place. This possibility cannot totally be ruled out
-
(27) Kotowski,M.;vanEldik, R. 1nZnorganicHighA.essure Chemistry. Kinetics and Mechanism; van Eldik, R., Ed.; Eleevier: Amsterdam, Oxford, New York, Tokyo, 1986; p 219. (28) Sylvestre, J.; Hoffmann,R. Helu. Chim. Acta 1985,68, 1461.
Organometallics, Vol. 12, No. 4, 1993 1393 Scheme I1
J r 7 ~
,
JrC\
-.”
T
!
fast
s
i”i
in light of results described in this paper, though it seems
to be less likely than the mechanism shown in Scheme 11. Conclusions. This study has shown that prior to the rate-limiting insertion into Pd-C bonds of halo-bridged palladacycles, alkynes can precoordinate Pd(I1) via the bridge-opening route. The insertion reaction itself seems to be to a significant extent nonconcerted, and the nucleophilic attack of the C1 carbon at the coordinated alkyne, which might be to a significant extent +bound, plays an important role. Similar consideration of the elementary steps of the second insertion gives understanding to how the cis to trans isomerization of the alkene C=C bond may take place.
Acknowledgment. A stipend from the Alexander von Humboldt Foundation for A.D.R. is gratefully acknowledged. R.v.E. gratefully acknowledges financial support from the Deutsche Forshungsgemeinschaft and the Vokswagen-Stiftung. Supplementary Material Available: Tables of positional parametersof the hydrogen atoms, bond lengths and angles, and anisotropic thermal parameters for Sb (5 pages). Ordering information is given on any current masthead page. OM9206719
